The author reviewed the question of late side effects of cancer treatment. A late side effect was defined as an unwanted and deleterious effect observed for the first time 12 months or longer after treatment. In a survey of articles published by The New England Journal of Medicine and The Lancet from 1968 to 1988, nine categories of late side effects were identified: second cancers, hormonal and reproductive effects, effects on the immunologic system, heart disease, effects on kidney and urinary bladder, effects on gastrointestinal organs, neurologic and psychological effects, pulmonary toxicity, and osteonecrosis. The induction of second cancers is a particularly important late side effect of cancer treatment. Several epidemiologic studies demonstrated increased risks of leukemia and solid tumors in patients exposed to radiotherapy. Large increases in leukemia risk have also been observed after chemotherapy with alkylating agents. However, several research questions remain unanswered, including the duration of the excess cancer risk after treatment, and the independent effect of various drugs on second cancer risk.